The present invention relates to image intensifier tubes used in the field of imaging or inspection. The present invention finds particular application in conjunction with fluoroscopic imaging systems and will be described with particular reference thereto. However, it should be appreciated that the present invention also finds application in areas wherein images, resulting from the conversion of electromagnetic radiation of a wide range of wavelengths or particulate matter, such as thermal neutron, alpha particles, high energy electron beams or the like, are viewable on a CRT.
One type of conventional fiuoroscopic imaging system is comprised of a source of penetrative radiographic energy which propagates a beam of radiation through an object to be imaged. The beam passing through the object is selectively attenuated by internal structures within the object, according to the density of the internal structure, such that the beam exiting the object contains a radiographic representation of the objects internal structure. The input face of an image intensifier tube is disposed in the path of the exiting beam such that the radiation beam containing the radiographic representation impinges thereon. The image intensifier tube converts the radiographic representation into a viewable light image on the output end thereof. The image intensifier tube is comprised of an evacuated tube having a circular input screen and a smaller diameter circular output screen. The input screen is comprised of a fluorescent material, such as cesium iodide. The radiation beam Impinging on the fluorescent material is absorbed thereby and a portion of the absorbed energy Is converted into light. Closely adjacent the florescent material Is a photocathode which absorbs the light emitted by the interaction of the radiation beam and the fluorescent material. The photocathode emits free electrons into the evacuated tube In response to the interaction of the light on the photocathode. The electron density of the free electrons emitted from point-to-point on the photocathode screen corresponds to the intensity of the light reaching the photocathode at each point thereon. An electrical potential is applied between the Input screen and the output screen for accelerating the free electrons towards the output end of the tube where they strike the output screen. A phosphor on the output screen converts the impinging electrons into visible light. While the image at the output face of the intensifier tube is directly viewable it is typically not useable fiber diagnostic viewing because of Its relatively small output diameter.
To overcome this shortcoming, the input side of a video camera tube is optically coupled to the output face of the intensifier tube. The video camera is comprised of an evacuated camera tube and associated electronics. The camera tube and associated electronics convert the viewable light image at the output of the intensifier tube Into a electronic signal equivalent of the viewable light image in a known manner. The electronic signal from the video camera is communicated to a cathode ray tube (CRT) for viewing of the image or to a storage device, for subsequent electronic manipulation thereof by image processing algorithms as are know in the art. The viewable image produced on the CRT from the electronic signal is an enlarged version of the image at the image intensifier output.
Electronic sensor arrays, such as semiconductor charge coupled devices (CCD), have started replacing video camera tubes. These sensors are typically smaller in size, less expensive, consume less power, require less interfacing hardware and tend to have a longer useful life than the video camera tubes they replace. CCD arrays are comprised of a plurality of light receiving elements formed into a array of rows and columns. In operation, the light receiving side of the CCD is placed in close proximity to the image intensifier output such that the entire diameter of the image intensifier output is completely viewable by the CCD array. Each element in the array receives light from a closely adjacent portion of the intensifier tube output screen and converts the light into an electrically equivalent output thereof. At predetermined intervals, control electronics connected between the CCD array and the CRT scan the electrical output of each element in the array in synchronization with the scan of the display on the CRT such that the electrical output of the array elements are reproduced as a viewable image, representative of the image intensifier output.
A number of CCD arrays of varying shape are commercially available. A particularly popular shape for a CCD array is rectangular wherein the width-to-height ratio is 4:3. This aspect ratio is popular because the CRT of commercial television equipment have the same aspect ratio. In this application the entire CCD array is used to view an image which can be reproduced on the television CRT without any aspect ratio conversion. For each shape of CCD array a plurality of light receiving element densities are available to provide choice of image resolution capability to the user. A greater light receiving element density corresponds to a greater resolution of the resultant image being available to the CRT. Generally, as the light receiving element density increases the CCD arrays become more expensive and difficult to obtain. Moreover, CCD arrays having an aspect ratio other than 4:3 are generally less available and thus, more expensive. Accordingly, in fluoroscopic imaging applications, it is desirable to utilize CCD arrays having a 4:3 aspect ratio and having a light receiving element density that corresponds to element densities used with commercial television equipment. One problem with using 4:3 aspect ratio CCD arrays in fiuoroscopic imaging is that convention dictates that the circular intensifier output be completely reproduced on the CRT. Using a 4:3 aspect ratio CCD to view the circular output of an image intensifier tube results in a maximum CCD utilization of about 59%, the remaining 41% going unused.
One apparatus that uses more of the viewing capacity of a 4:3 aspect ratio CCD is disclosed in U.S. Pat. No. 4,857,724 to Snoeren and having an issue date of Aug. 15, 1989. The '724 patent discloses an anamorphic lens disposed between the circular intensifier output screen and the 4:3 aspect ratio CCD such that the output image is horizontally stretched in a direction along the longer axis of the CCD to utilize more of the array elements. One problem with stretching the image in this manner is that if this image were directly transferred to a 4:3 aspect ratio CRT the viewable image would be horizontally stretched along its longer axis. However, by simultaneously increasing the horizontal read scan speed of the CCD and synchronizing the start of horizontal read scan of the CCD to the horizontal scan of the CRT output image scan, the resultant viewable image can be converted into an undistorted circular image. In this manner a 4:3 aspect ratio CCD, used in conjunction with a conventional image intensifier tube, can be effectively used to present a conventional circular image on a commercially available CRT.
One problem with this approach is that an anamorphic lens is comprised of a plurality of Individual external optical lenses arranged in optical alignment to change the aspect ratio of the image. Because a plurality of lenses are required this approach is optically inefficient thereby causing a corresponding decrease in the overall efficiency of the system. Accordingly, a higher does rate is required to obtain the image contrast necessary for diagnostic imaging. Moreover, the prior approach results in the size of system being rather large, with an optical length being over 170 mm.
The present invention provides a new and improved way to present a 4:3 aspect ratio image to a 4:3 aspect ratio CCD input while avoiding the need to place an anamorphic lens between the image intensifier output screen and the video camera input screen.